TypeScript Types

Any

• Every untyped variable whose type cannot be inferred, becomes implicitly any type.
• Any is a wild card type
• "top type"
• Implicit any typings are usually considered problematic
  • TypeScript not used properly
  • programmer forgot to assign type
• why the configuration rule noImplicitAny is recommended at all time

Never

• "bottom type"
• it will never work - can not happen
• have problem pushing anything into an array of never[]

Tuples

• structured set of data - comes with convention
• an array of fixed length
• example array of address

let bb: [number, string, string, number] = [
  123,
  "Fake Street",
  "Nowhere
]

• great way to return a set of data from function
• set and retrieve - not meant for updating - becareful in how we use tuples
  • set is typed checked all at once - correct usage
  • can not safely type push function - not to use array methods on tuples
• tuple values often require type annotations - need to be specific

Object Types

• all properties are required

• use optional operator (?)

• use @ts-check to check for type errors in your JavaScript

  // @ts-check
  

import { Bear } from './bear.model'; 
const bear = new Bear(3);
if (bear instanceof Bear) {
    console.log("Hello from TypeScript");
}

Return Types

import { Person } from './person.model';
function add(val1: number, val2: number): number {
    return val1 + val2;
}
function sayHello(person: Person): string {
    return `Say Hello to My Little Friend, ${person.firstName}!`    
}
function voidExample(): void {
    add(1,2);
}
function neverExample(): never {
}

to review

• generic types